1. Field of the Invention
The present invention relates to a capsulated toner.
2. Description of the Related Art
An electrophotographic image forming apparatus comprises components used for an image forming process mechanism, including: a photoreceptor, a charging section for charging a surface of the photoreceptor, an exposure section for irradiating the surface of the photoreceptor being charged with signal light to form thereon an electrostatic latent image corresponding to image information, a developing section for supplying a toner contained in a developer retained in a developer tank to the electrostatic latent image formed on the surface of the photoreceptor to form thereon a toner image, a transfer section provided with a transfer roller for transferring the toner image formed on the surface of the photoreceptor to a recording medium, a fixing section provided with a fixing roller for fixing the toner image onto the recording medium, and a cleaning section for cleaning the surface of the photoreceptor after the toner image has been transferred. In the electrophotographic image forming apparatus, the electrostatic latent image is developed using a one-component developer including a toner, or a two-component developer including a toner and a carrier, as a developer to form an image.
The electrophotographic image forming apparatus can form an image having fine image quality at high speeds and low costs, thereby being utilized for copying machines, printers, facsimiles and the like, along with remarkable popularization thereof in recent years. Simultaneously, requirements for the image forming apparatus have become all the more severe. For example, with the requirement of reduction of carbon-dioxide emissions for prevention of global warming, lower power consumption of the image forming apparatus has become a major issue. That is, the image forming apparatus requires a method for forming an image by fusing a toner mainly using a thermoplastic resin as a binder resin and fixing the toner to the recording medium, in which the thermoplastic resin has to be fused by heating the toner up to a temperature of around 100° C. or more. To heat the toner, a heating apparatus having a large power consumption, such as a heater, is used. In addition, the heating apparatus is set so as to continue a heating for resuming an image forming in a short time, by maintaining a temperature of the fixing section fixed even when the heating apparatus is in a ready and waiting state in which an image is not formed. Moreover, a few hundred to a few thousand of images are formed per day on one image forming apparatus. Therefore, in the image forming apparatus, power consumption required for fixing a toner is too much to ignore. Accordingly, a toner having a lower temperature (a fixing temperature) to be used for fixing the toner onto the recording medium has been achieved. As the toner having the lower fixing temperature, examples thereof include a toner using resin materials having lower glass transition temperatures, a lower softening temperature, and the like, as a binder resin, and a toner composed by dispersing a wax having a low melting point into a binder resin. However, there has been a problem in which these toners are excellent in low-temperature fixing ability onto the recording medium, but are insufficient with respect to storage stability thereof. For example, when a toner having a low melting point is retained for a long time in the developer tank, the toner is fused and attached to the photoreceptor, causing toner filming, and thus causing a defective image, or reduction of a lifetime of the photoreceptor. In addition, when the two-component developer is used, the toner filming onto a surface of a carrier also takes place, causing the defective image.
To solve the problems for the toner having a low melting point, there has been disclosed a capsulated toner achieved by a coating surface of a core particle including a colorant and a binder resin with a shell layer composed of fine resin particles (refer to Japanese Unexamined Patent Publication JP-A 5-107808 (1993) and Japanese Unexamined Patent Publication JP-A 5-181301 (1993). The capsulated toner disclosed in JP-A 5-107808 is a toner obtained by coating the surface of the core particle containing polyester with the shell layer composed of fine particles of a styrene resin and/or an acrylic resin obtained by soap-free emulsion polymerization. The capsulated toner disclosed in JP-A 5-181301 is a toner obtained by coating the surface of the core particle composed of a styrene-acrylic resin copolymer with the shell layer composed of fine particles of the styrene resin. In these patent documents, by using resin materials having comparatively lower glass transition temperatures or melting points as the binder resin contained in the core particle, and by using the fine particles composed of resins having relatively higher glass transition temperatures or melting points than those of the binder resin of the core particle as fine resin particles constituting the shell layer, the storage stability of the toner has been improved while maintaining the low-temperature fixing ability onto the recording medium. The styrene resin has an enhanced hydrophobic characteristic and a reduced moisture-absorption characteristic, providing an advantage that a change in chargeability due to moister absorption is decreased. However, the styrene resin has a high glass transition temperature of 100° C. or more, possibly decreasing the low-temperature fixing ability when used alone. The acrylic resin is advantageous in that its glass transition temperature can be adjusted by selecting a monomer. However, there has been a problem in which the chargeability thereof is changed by absorbing moisture at a high temperature and a high humidity, since molecules thereof include hydrophilic portions such as an ester bond and a hydroxyl group. Fine resin particles composed of polyester are also used other than the styrene resin and the acrylic resin, but polyester has the same problem as the acrylic resin.
Meanwhile, it has been previously proposed that a cycloolefin resin is used as a binder resin of a toner (refer to JP-A 2003-114546, for example). The cycloolefin resin has high transparency and is thus suitable for forming a color image, and has a low specific gravity and is thus capable of reduction of a toner consumption. In addition, the cycloolefin resin has no polar radical in molecules thereof and a low moisture-absorption characteristic, thus providing good charge stability. In addition, the use of the cycloolefin resin facilitates control of a glass transition temperature by selecting a kind of monomers. As described above, the cycloolefin resin has various kinds of advantages, and is thus useful as a binder resin of a toner. However, the cycloolefin resin has no polar radical in molecules thereof, and thereby has an advantage of exhibiting the good charge stability, but, on the other hand, has a problem of low adhesion to a sheet of paper. Accordingly, an image formed by using a toner containing the cycloolefin resin as a binder resin has a low fixing level to a sheet of paper, and a low print gloss.